Conventionally, two methods were known for the formation of a passive oxide film having chromium oxide as a chief component thereof on a stainless steel surface: the dry method, in which, after directly reacting stainless steel with oxygen gas, the oxidized steel was reduced with hydrogen gas and heat treated with an inert gas such as argon after reduction, and thereby, a passive film having chromium oxide as a chief component thereof was formed; and the wet method, in which the steel was etched using a chemical such as nitric acid or the like, and chromium oxide was obtained. A diagram of the processes of the dry method is shown in FIG. 5(b).
In FIG. 5(b), (1) indicates a baking process which removes moisture adhering to the stainless steel surface, and moisture released by the stainless steel surface. (2) indicates an oxidation process which is conducted in an oxygen atmosphere. The film obtained by this oxidation process is a passive oxide film having iron oxide as a chief component thereof. (3) indicates a reducing process in which the iron oxide is reduced in a hydrogen atmosphere in order to obtain chromium oxide. (4) indicates a heat treatment process in an inert gas atmosphere for the purpose of conversion to a film having chromium oxide as the chief component thereof. In this way, in accordance with the dry method, the formation of the chromium oxide is conducted by means of independent oxidation and reduction reactions, so that the period required for the processes is long.
FIG. 6 shows data relating to moisture released at normal temperatures from passive oxide films obtained by means of the wet method and the dry method, as measured by APIMS. As is clear from FIG. 6, in contrast to the passive oxide film formed in accordance with the dry method, which ceased giving off moisture after several minutes, the passive oxide film obtained in accordance with the wet method continued to give off moisture even after the passage of 100 minutes. In this way, the passive oxide film obtained in accordance with the wet method contained a large moisture component, so that if the moisture were not removed, such a passive oxide film could not be used in semiconductor production apparatuses, which must be free of outside gasses, and heat treatment such as baking or the like was necessary, so that in the same manner as with the dry method, considerable time was required.
The present invention has as an object thereof to provide a method of forming a passive oxide film having chromium oxide as a chief component thereof which is capable of easily forming a passive oxide film having chromium oxide as a chief component thereof, and to provide a stainless steel having a passive oxide film having chromium oxide as a chief component thereof.